The monitoring and control of gases during combustion is of increasing importance. Therefore, it is necessary to develop inexpensive, robust and reliable sensors having high sensitivity and selectivity. The sensors can be used to control the combustion process, and to detect when inefficient combustion is occurring, when toxic gases are being released and when the combustion unit requires tuning.
Two types of solid state sensor have been used for the determination of the oxygen content of gases. The electrolyte that is usually used is stabilised zirconia which comprises a solid solution of zirconium oxide and calcium oxide, magnesium oxide or yttrium oxide. In one mode, the potential is measured across the electrolyte when the gas to be measured is on one side of the electrolyte and a reference gas, of known oxygen partial pressure, is on the other side of the electrolyte. In another mode, the electrolyte is separated from gas by a pinhole or a porous layer, a potential is applied and the current measured.
In the potentiometric mode, the potential is measured across the electrolyte and the gas composition on either side of the electrolyte is given by the well-known Nernst equation:
-ZEF=RT lnp'.sub.x2.sub./p".sub.x2 (1)
where Z is the charge carried, E is the measured potential, F is Faraday's constant, R is the gas constant, and P'.sub.x2 and P ".sub.x2 are the pressures of gas on either side of the electrolyte. It can be appreciated that, if the potential is measured and the gas composition is known on one side of the electrolyte, the gas composition on the other side of the electrolyte can be determined. This approach has been used to great effect in controlling the internal combustion engine and burners, around the stoichiometric ratio, where the gas goes from highly reducing to highly oxidising over a small range of air/fuel ratio. Under these conditions, the oxygen pressure changes dramatically and a large potential change results.
However, for some engines and many combustion systems, it is preferable to operate in a region where excess air is used and the residual gases contain free oxygen. In such a lean-burn system, unlike the circumstances occurring around the stoichiometric region, there is very little change in oxygen partial pressure, which results in very small changes in the Nernst potential, given by equation (1). In many cases, the predicted change in potential is less than the scatter in the reading.
In order to circumvent this problem, an amperometric sensor has been used which comprises an electrolyte plate or cup, with an electrode on each face, separated from the gas source by a pinhole or porous coating. On the application of a voltage across the electrolyte, via the electrodes, oxygen ionises to ions which pass through the electrolyte to where the ions are discharged to oxygen gas at the other electrode. The supply of oxygen to the ionising electrode is controlled by the supply of oxygen via the pinhole or the porous coating and, therefore, the measured current is dependent on the supply of oxygen.
As the concentration at the electrode surface is zero (as all the oxygen is ionised), the current is linearly dependent on the oxygen concentration in the gas external to the pinhole or the porous coating. Furthermore, the measured current is usually found to be independent of the applied potential. However, it is extremely difficult to maintain the size of the pore or the porosity to any degree of reliability. For this reason, these sensors are not widely used, and the development of lean-burn engines and combustion systems has been restricted.